The USA's long battle against air pollution isn't over yet, as air quality improvements are slowing down

Following five decades of progress in cleaning up our air, U.S. pollution gains have slowed significantly in recent years, a new study concludes.

The surprising result means that it may be more difficult than previously thought for the U.S. to achieve its goal of cleaner air, scientists said.

"Although our air is healthier than it used to be in the '80s and '90s, air quality in the U.S. is not progressing as quickly as we thought," said National Center for Atmospheric Research (NCAR) scientist Helen Worden, a study co-author. "The gains are starting to slow down."

The two pollutants studied were carbon monoxide and nitrogen oxide, both of which contribute to ground-level ozone (smog). Smog forms on warm, sunny days and is made worse from chemicals that come from car and truck tailpipes and from power plant and industrial smokestacks. Warmer temperatures make ozone more likely to form.

Exposure to elevated ozone can lead to coughing and difficulty breathing, and make respiratory diseases such as asthma worse, according to University of Maryland air scientist Ross Salawitch.

The study said pollution levels from nitrogen oxide fell 7% from 2005 to 2009, but only dropped 1.7% from 2011 to 2015. That translates to a 76% slowdown between the late 2000s and early 2010s.

This contrasts sharply with a more rosy outlook from the Environmental Protection Agency, which said the slowdown in reductions was only 16%.

"We were surprised by the discrepancy between the estimates of emissions and the actual measurements of pollutants in the atmosphere," said Zhe Jiang, lead author of the study, who previously worked at NCAR and is now with the University of Science and Technology of China

"These results show that meeting future air quality standards for ozone pollution will be more challenging than previously thought," Jiang said.

One reason for the slowdown, scientists say, is additional pollutants from sources such as industrial, residential, and commercial boilers and off-road vehicles. Another reason is due to slower-than-expected reductions in emissions from heavy-duty diesel trucks.

The results also show the EPAs computer models overestimate how clean the air really is, according to the University of North Carolinas Jason West, who wasnt part of the study.

The slowdown in pollution reduction was most noteworthy across the eastern U.S., which was one of several signs the pollution wasn't coming across the Pacific Ocean from Asia.

In the study, scientists used a combination of data from satellites, computer simulations and ground-based air quality monitoring stations to reach their conclusions.

The study appeared in the Proceedings of the National Academy of Sciences.